The state of the art is discussed in the following with respect to FIGS. 1a and 1b of the drawing.
In the area concerning the invention, two-level control processes having a PI-characteristic are generally utilized with the control factor FR being supplied as an actuating variable with which a precontrol value is multiplied for the actuating signal of a fuel-metering arrangement. As a rule, the fuel-metering arrangement is an injection-valve arrangement and the precontrol values are preliminary injection time durations. The lambda value is used as a control value whose actual value is determined in the exhaust gas of the engine by means of a lambda probe displaying the performance referred to above. The lambda actual value measured at a predetermined point in time is assigned to a fuel injection quantity which was adjusted at a point in time which is earlier by a dead time T. This adjustment of the fuel quantity is made by means of the control method. Because of this dead time T, a control oscillation of the control factor FR is introduced which is illustrated in FIG. 1a.
FIG. 1a also shows another measure which is often used, namely, the P-component (proportional component) is greater in the direction of making the mixture rich than in the direction of making the latter lean. This serves to make the adjustment of the mean lambda value somewhat smaller (richer) than unity (1) in order to attain the optimal operating point of the three-way catalyzer which is conventionally used. In this respect, reference may be made to U.S. Pat. No. 4,210,106 which is incorporated herein by reference.
It is known that the probe voltage US oscillates at times with a higher frequency about a reference voltage UREF than can be derived from the control oscillation period. This oscillation of a higher frequency occurs, for example, because of scattering in the air number of different volumes of air or different combustions. Such effects are often included under the common term of "chemical noise". It should be noted that the effects are especially pronounced in certain speed and load ranges which are dependent upon the engine.
In the diagram of FIG. 1a, it is assumed that the control factor FR will vary with an amplitude of approximately 4% deviation from its average mean value lying just below unity (1) because of fluctuations in the actual value of lambda caused by the normal control oscillations. It is further assumed that dynamic effects lead to a lambda value fluctuation of approximately 1%. This means that when the multiplication factor FR is just in the range between approximately 0.99 and 1.01 during its control oscillations, the influence of the dynamic effects on the lambda-value measurement can outweigh the influence of the control oscillations so that this can therefore lead to an oscillation of the probe voltage US with increased frequency.
The time range within which the control factor FR passes through the range between 1.01 and 0.99 for a correct oscillatory performance of the control unit is indicated in FIG. 1a by reference designation TI. It is assumed that in the second control pulse in FIG. 1a, a the second half of dynamic performance of the lambda value occurs, for example, because the rotational speed has entered into a critical range thereof. The dynamic performance of the lambda value at the air-intake end of the engine during the time duration TI is delayed at the lambda probe by a dead time T as shown all the way to the right in FIG. 1b. Here it can be seen how the probe voltage US oscillates about the reference voltage UREF at a relatively high frequency. With each pass-through of the probe voltage through the value of the reference voltage UREF, this oscillation leads to a change in the operating direction of the control process as shown all the way to the right in FIG. 1a. The continuous alternations lead to a rapid increase of the control factor FR in the direction "rich" because, as explained above, the P-component in the direction "rich" is greater than the P-component in the direction "lean".
The last-mentioned rapid increase of the control factor FR is prevented in the lambda control device MOTRONIC (trademark of Robert Bosch GmbH) in that the time duration between two pass-throughs of the probe voltage US through the reference voltage UREF is controlled. As soon as this time duration drops below a threshold such as the speed-dependent dead time T, the assumption is made that the above-mentioned dynamic effects become effective. The P-component is then set for both control directions to that value which applies during proper operation only to the direction "rich". In this way, the control factor FR no longer changes rapidly in one direction; however, an uncontrollable drift of this factor does occur since the reference voltage is not passed through because of the I-component (integral-action component) in the control factor; instead, this reference voltage UREF is passed through because of the P-component thereof.